Tissue impedance measurement using a secondary frequency

ABSTRACT

A microwave ablation system includes a generator including a first energy source, a second energy source and a diplexer, the diplexer multiplexes a first energy from the first energy source and a second energy from the second energy source. The system also includes a cable including a center conductor and an outer sheath where the multiplexed energy is transmitted through the center conductor. In addition an antenna is provided that is operable to receive the multiplexed energy from the center conductor and to deliver the multiplexed energy to a region of tissue. The outer sheath acts as a return path of the second energy to the second energy source. A sensor is also provided that measures at least one parameter of the second energy generated by the second energy source and the second energy returned from the region of tissue.

CROSS REFERENCE TO RELATED APPLICATION

The present application is a Divisional Application claiming the benefit of and priority to U.S. application Ser. No. 12/468,718, filed on May 19, 2009, by Robert J. Behnke, entitled “TISSUE IMPEDANCE MEASUREMENT USING A SECONDARY FREQUENCY”, the entire contents of which being incorporated by reference herein.

BACKGROUND

1. Technical Field

The present disclosure relates generally to microwave ablation procedures that utilize microwave surgical devices having a microwave antenna which may be inserted directly into tissue for diagnosis and treatment of diseases. More particularly, the present disclosure is directed to measuring tissue impedance during a microwave ablation procedure.

2. Background of Related Art

In the treatment of diseases such as cancer, certain types of cancer cells have been found to denature at elevated temperatures (which are slightly lower than temperatures normally injurious to healthy cells.) These types of treatments, known generally as hyperthermia therapy, typically utilize electromagnetic radiation to heat diseased cells to temperatures above 41° C., while maintaining adjacent healthy cells at lower temperatures where irreversible cell destruction will not occur. Other procedures utilizing electromagnetic radiation to heat tissue also include ablation and coagulation of the tissue. Such microwave ablation procedures, e.g., such as those performed for menorrhagia, are typically done to ablate and coagulate the targeted tissue to denature or kill the tissue. Many procedures and types of devices utilizing electromagnetic radiation therapy are known in the art. Such microwave therapy is typically used in the treatment of tissue and organs such as the prostate, heart, liver, lung, kidney, and breast.

One non-invasive procedure generally involves the treatment of tissue (e.g., a tumor) underlying the skin via the use of microwave energy. The microwave energy is able to non-invasively penetrate the skin to reach the underlying tissue. However, this non-invasive procedure may result in the unwanted heating of healthy tissue. Thus, the non-invasive use of microwave energy requires a great deal of control.

Presently, there are several types of microwave probes in use, e.g., monopole, dipole, and helical. One type is a monopole antenna probe, which consists of a single, elongated microwave conductor exposed at the end of the probe. The probe is typically surrounded by a dielectric sleeve. The second type of microwave probe commonly used is a dipole antenna, which consists of a coaxial construction having an inner conductor and an outer conductor with a dielectric junction separating a portion of the inner conductor. The inner conductor may be coupled to a portion corresponding to a first dipole radiating portion, and a portion of the outer conductor may be coupled to a second dipole radiating portion. The dipole radiating portions may be configured such that one radiating portion is located proximally of the dielectric junction, and the other portion is located distally of the dielectric junction. In the monopole and dipole antenna probe, microwave energy generally radiates perpendicularly from the axis of the conductor.

The typical microwave antenna has a long, thin inner conductor that extends along the axis of the probe and is surrounded by a dielectric material and is further surrounded by an outer conductor around the dielectric material such that the outer conductor also extends along the axis of the probe. In another variation of the probe that provides for effective outward radiation of energy or heating, a portion or portions of the outer conductor can be selectively removed. This type of construction is typically referred to as a “leaky waveguide” or “leaky coaxial” antenna. Another variation on the microwave probe involves having the tip formed in a uniform spiral pattern, such as a helix, to provide the necessary configuration for effective radiation. This variation can be used to direct energy in a particular direction, e.g., perpendicular to the axis, in a forward direction (i.e., towards the distal end of the antenna), or combinations thereof.

Invasive procedures and devices have been developed in which a microwave antenna probe may be either inserted directly into a point of treatment via a normal body orifice or percutaneously inserted. Such invasive procedures and devices potentially provide better temperature control of the tissue being treated. Because of the small difference between the temperature required for denaturing malignant cells and the temperature injurious to healthy cells, a known heating pattern and predictable temperature control is important so that heating is confined to the tissue to be treated. For instance, hyperthermia treatment at the threshold temperature of about 41.5° C. generally has little effect on most malignant growth of cells. However, at slightly elevated temperatures above the approximate range of 43° C. to 45° C., thermal damage to most types of normal cells is routinely observed. Accordingly, great care must be taken not to exceed these temperatures in healthy tissue.

In the case of tissue ablation, a high radio frequency electrical current in the range of about 500 mHz to about 10 gHz is applied to a targeted tissue site to create an ablation volume, which may have a particular size and shape. Ablation volume is correlated to antenna design, antenna performance, antenna impedance and tissue impedance. The particular type of tissue ablation procedure may dictate a particular ablation volume in order to achieve a desired surgical outcome. By way of example, and without limitation, a spinal ablation procedure may call for a longer, narrower ablation volume, whereas in a prostate ablation procedure, a more spherical ablation volume may be required.

Microwave ablation devices utilize thermocouples to determine when an ablation is complete. When the thermocouple reaches threshold temperature the ablation procedure is completed. Tissue impedance can also be used to determine when the ablation procedure is completed. Because ablated tissue blocks electrical signals due to non-conductive dessicated tissue, the tissue impedance can determine when the ablation procedure is completed. During application of energy, the current and voltage applied to the tissue can be measured and used to calculate impedance and the calculated impedance is stored. Based upon a function of the impedance it is determined whether the ablation procedure is complete.

During an ablation procedure, if only one antenna is used, there is no easy way to determine a change in tissue impedance. The inability to determine the change in tissue impedance is due to losses in the coaxial cable used to deliver energy to the antenna. The coaxial cable can make it difficult to measure any reflection from the antenna that could be used to determine the tissue impedance.

Additionally, antennas may use a coolant to provide improved ablation volume and shape. Any suitable medium may be used as a coolant such as deionized water, sterilized water, or saline. The coolant may have dielectric properties which may provide improved impedance matching between an antenna probe and tissue. Impedance matching is the practice of setting the probe impedance to the tissue impedance in order to maximize the power transfer and minimize reflections from the load. Due to the minimized reflections, if a water or dielectric buffer cooled antenna is used, the change in tissue impedance may be immeasurable.

SUMMARY

The present disclosure provides a microwave ablation system. The microwave ablation includes a generator including a first energy source, a second energy source and a diplexer, the diplexer operable to multiplex a first energy from the first energy source and a second energy from the second energy source. A cable is also provided that includes a center conductor and an outer sheath. The multiplexed energy is transmitted through the center conductor. Also, an antenna operable to receive the multiplexed energy from the center conductor and to deliver the multiplexed energy to a region of tissue is provided. The outer sheath acts as a return path of the second energy to the second energy source. A sensor is also provided that measures at least one parameter of the second energy generated by the second energy source and the second energy returned from the region of tissue.

The present disclosure also provides another microwave ablation system. The microwave ablation system includes a generator including a first energy source, a second energy source and a diplexer, the diplexer operable to multiplex a first energy from the first energy source and a second energy from the second energy source. A cable is also provided that includes a center conductor and an outer sheath. The multiplexed energy is transmitted through the center conductor. Also, an antenna operable to receive the multiplexed energy from the center conductor and to deliver the multiplexed energy to a region of tissue is provided. Additionally, a return pad is provided that is operable to receive the second energy outputted from the antenna. The return pad transmits the second energy to the second energy source. A sensor is also provided that measures at least one parameter of said second energy generated by said second energy source and the second energy returned from said return pad.

The present disclosure also provides a method of ablating tissue using microwave energy. The method includes generating microwave energy, generating radio frequency (RF) energy, multiplexing the microwave energy and the RF energy and outputting the multiplexed energy to a tissue region. RF energy is returned from the tissue region and at least one parameter of the generated RF energy and the RF energy returned from said tissue region is measured. Based on the measured parameter the level of microwave energy that is generated is controlled.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other aspects, features, and advantages of the present disclosure will become more apparent in light of the following detailed description when taken in conjunction with the accompanying drawings in which:

FIG. 1 shows a representative diagram of a variation of a microwave antenna assembly in accordance with an embodiment of the present disclosure;

FIG. 2 shows a cross-sectional view of a representative variation of a distal end of microwave antenna assembly in accordance with an embodiment of the present disclosure;

FIG. 3 shows a cross-sectional view of a representative variation of a proximal end of microwave antenna assembly in accordance with an embodiment of the present disclosure;

FIGS. 4A-4D show perspective views of the distal portion of a microwave antenna in various stages of assembly in accordance with an embodiment of the present disclosure;

FIG. 5 is a schematic diagram of a microwave ablation system with a bipolar RF energy source according to an embodiment of the present disclosure;

FIG. 6 is a diagram of a bipolar RF return path according to an embodiment of the present disclosure;

FIG. 7 is a schematic diagram of a microwave ablation system with a monopolar RF energy source according to an embodiment of the present disclosure; and

FIG. 8 is a diagram of a monopolar RF return path according to an embodiment of the present disclosure.

DETAILED DESCRIPTION

Particular embodiments of the present disclosure are described hereinbelow with reference to the accompanying drawings; however, it is to be understood that the disclosed embodiments are merely exemplary of the disclosure and may be embodied in various forms. Well-known functions or constructions are not described in detail to avoid obscuring the present disclosure in unnecessary detail. Therefore, specific structural and functional details disclosed herein are not to be interpreted as limiting, but merely as a basis for the claims and as a representative basis for teaching one skilled in the art to variously employ the present disclosure in virtually any appropriately detailed structure. Like reference numerals may refer to similar or identical elements throughout the description of the figures.

Electromagnetic energy is generally classified by increasing energy or decreasing wavelength into radio waves, microwaves, infrared, visible light, ultraviolet, X-rays and gamma-rays. As used herein, the term “microwave” generally refers to electromagnetic waves in the frequency range of 300 megahertz (MHz) (3×10⁸ cycles/second) to 300 gigahertz (GHz) (3×10¹¹ cycles/second). As used herein, the term “RF” generally refers to electromagnetic waves having a lower frequency than microwaves. The phrase “ablation procedure” generally refers to any ablation procedure, such as microwave ablation or microwave ablation assisted resection. The phrase “transmission line” generally refers to any transmission medium that can be used for the propagation of signals from one point to another.

FIG. 1 shows an embodiment of a microwave antenna assembly 100 in accordance with one embodiment of the present disclosure. Antenna assembly 100 includes a radiating portion 12 that is connected by feedline 110 (or shaft) via cable 15 to connector 16, which may further connect the assembly 10 to a power generating source 28, e.g., a microwave or RF electrosurgical generator. Assembly 100, as shown, is a dipole microwave antenna assembly, but other antenna assemblies, e.g., monopole or leaky wave antenna assemblies, may also utilize the principles set forth herein. Distal radiating portion 105 of radiating portion 12 includes a tapered end 120 which terminates at a tip 123 to allow for insertion into tissue with minimal resistance. It is to be understood, however, that tapered end 120 may include other shapes, such as without limitation, a tip 123 that is rounded, flat, square, hexagonal, or cylindroconical.

An insulating puck 130 is disposed between distal radiating portion 105 and proximal radiating portion 140. Puck 130 may be formed from any suitable elastomeric or ceramic dielectric material by any suitable process. In embodiments, the puck 130 is formed by overmolding from polyether block amide (e.g., Pebax®), polyetherimide (e.g., Ultem® and/or Extem®), polyimide-based polymer (e.g., Vespel®), or ceramic. As best illustrated in FIG. 2, puck 130 includes coolant inflow port 131 and coolant outflow port 133 to respectively facilitate the flow of coolant into, and out of, coolant chamber 148 of trocar 122 as further described hereinbelow.

With reference now to FIGS. 2, 3, and 4A-4D, distal radiating portion 105 includes a trocar 122 having a generally cylindroconical shape. Proximal radiating portion 140 includes a proximal antenna member 128 having a generally cylindrical shape. Additionally or alternatively, proximal antenna member 128 may have a generally square or hexagonal shape. Trocar 122 and proximal antenna member 128 may be formed from a variety of biocompatible heat resistant conductive materials suitable for penetrating tissue, such as without limitation, stainless steel. Antenna assembly 110 includes a coaxial transmission line 138 having, in coaxial disposition, an inner coaxial conductor 150, an intermediate coaxial dielectric 132, and an outer coaxial conductor 134. Nominally, coaxial transmission line 138 has an impedance of about 50 ohms. Inner coaxial conductor 150 and outer coaxial conductor 134 may be formed from any suitable electrically conductive material. In some embodiments, inner coaxial conductor 150 is formed from stainless steel and outer coaxial conductor 132 is formed from copper. Coaxial dielectric 132 may be formed from any suitable dielectric material, including without limitation, polyethylene terephthalate, polyimide, or polytetrafluoroethylene (PTFE) (e.g., Teflon®, manufactured by E. I. du Pont de Nemours and Company of Wilmington, Del., United States). Inner coaxial conductor 150 may be electrically coupled to trocar 122 and outer coaxial conductor 134 may be electrically coupled to proximal antenna member 128.

A longitudinal opening 146 is defined within trocar 122 and opens to a proximal end thereof, and defines a cooling chamber 148 and a threaded section 145 within trocar 122. Cooling chamber 148 may have a generally cylindrical shape and, additionally or alternatively, may have a stepped, tapered, conical, or other shape that is generally dimensioned in accordance with the shape of the tapered end 120 of the cylindroconical profile of trocar 122 to permit the flow of coolant to more effectively reach the distal regions of trocar 122. Additionally or alternatively, cooling chamber may have a square, hexagonal, or any suitable shape. Additionally, the dielectric properties of sterile water or saline flowing through cooling chamber 148 may enhance the overall ablation pattern of antenna 100.

A coolant inflow tube 126 is in operable fluid communication at a proximal end thereof with a source of cooling fluid (not explicitly shown), and, at a distal end thereof, coolant inflow tube 126 is in fluid communication with cooling chamber 146 to provide coolant thereto. Coolant inflow tube 126 may be formed from any suitable material, e.g., a polymeric material, such as without limitation, polyimide. In an embodiment, coolant inflow tube 126 passes through coolant inflow port 131. In some embodiments, a coolant outflow channel 136 may be provided to facilitate removal of coolant from cooling chamber 146, through antenna assembly 100, to a collection reservoir (not explicitly shown). The coolant may be any suitable fluid, such as without limitation water, sterile water, deionized water, and/or saline.

Threaded section 145 of trocar 122 is configured to receive trocar screw 144. Trocar screw 144 includes at the proximal end thereof an opening 143 defined therein that is configured to accept the distal end of inner coaxial conductor 150. In embodiments, distal end of inner coaxial conductor 150 is fixed within opening 143 by any suitable manner of electromechanical attachment, such as without limitation welding, brazing, and/or crimping. As seen in FIG. 4A, an inflow groove 147 and an outflow groove 149 are disposed longitudinally through the threaded portion of trocar screw 144 to respectively facilitate the flow of coolant into, and out of, cooling chamber 148. Inflow groove 147 and outflow groove 149 may be configured to accommodate the insertion of coolant inflow tube 126 and/or a corresponding outflow tube (not explicitly shown). A return path 156 in the antenna assembly may additionally or alternatively provide an exit conduit for the cooling fluid.

In the illustrated embodiment, trocar 122 and proximal antenna member 128 include a dielectric coating 121, 127, respectively, on the respective outer surfaces thereof. The dielectric coating 121, 127 may include any suitable dielectric material, such as without limitation, ceramic material. In some embodiments, dielectric coating 121, 127 may be formed from titanium dioxide and/or zirconium dioxide. Dielectric coating 121, 127 may be applied to trocar 122 and/or proximal antenna member 128 by any suitable process, for example without limitation, plasma spraying or flame spraying. In embodiments, dielectric coating 121, 127 has a thickness in the range of about 0.005 inches to about 0.015 inches. During an ablation procedure, the dielectric coating 121, 127 may provide improved dielectric matching and/or improved dielectric buffering between the antenna and tissue, which may enable the use of higher power levels, which, in turn, may enable a surgeon to achieve greater ablation rates resulting in increased ablation size, reduced operative times, and/or improved operative outcomes.

An outer jacket 124 is disposed about the outer cylindrical surface of antenna assembly 100, e.g., the distal radiating portion 105, puck 130, and proximal radiating section 140. Outer jacket 124 may be formed from any suitable material, including without limitation polymeric or ceramic materials. In some embodiments, outer jacket 124 is formed from PTFE. Outer jacket 124 may be applied to antenna assembly 100 by any suitable manner, including without limitation, heat shrinking.

Continuing with reference to FIGS. 4A-4D, a method of manufacturing antenna assembly 100 is disclosed wherein inner coaxial conductor 150 is inserted into opening 143 of trocar screw 144. Inner coaxial conductor 150 is electrically fixed to trocar screw 144 by any suitable manner of bonding, such as without limitation, laser welding, brazing, or crimping. The coaxial transmission line 138 and trocar screw 144 subassembly is placed in a mold (not explicitly shown), such as without limitation an injection micro-mold, that is configured to overmold the puck 130. Advantageously, inflow groove 147 and outflow groove 149 are aligned with mold features (not explicitly shown) corresponding to coolant inflow port 131 and coolant outflow port 133 such that, when molded, a continuous fluid connection is formed between inflow groove 147 and coolant inflow port 131, and between outflow groove 149 and outflow port 133.

Puck material, e.g., ceramic, Pebax®, Ultem®, Extem®, Vespel®, or any suitable polymer having dielectric properties, is shot into the mold, allowed to cool/and or set, and subsequently released from the mold to form an assembly that includes puck 130, trocar screw 144 and coaxial transmission line 138 as best illustrated in FIG. 4B. The formed puck 130 includes a center section 137 having an outer diameter corresponding to the outer diameters of trocar 122 (inclusive of the thickness of dielectric coating 121) and/or proximal antenna member 128 (inclusive of the thickness of dielectric coating 127). Puck 130 further includes a distal shoulder 141 having an outer diameter corresponding to the inner diameter of trocar 122, and a proximal shoulder 139 having an outer diameter corresponding to the inner diameter of proximal antenna member 128.

Trocar 122 may then be threaded onto trocar screw 144 to form the distal radiating section 120, as best shown in FIG. 4C. Inflow tube 126 may then be inserted into coolant inflow port 131. Proximal antenna member 128 may then be positioned against puck 130 such that the distal end of proximal antenna member 128 engages the proximal shoulder of puck 130, thus forming a sealed proximal radiation section 140.

Tension may be applied to inner coaxial conductor 150 and/or dielectric 132 in a proximal direction, thereby drawing together distal radiating section 105, puck 130, and proximal radiating section 140, and placing puck 130 in a state of compression. Inner coaxial conductor 150 and/or dielectric 132 may be fixed in a state of tension at an anchor point 151, by any suitable manner of fixation, including without limitation spot welding, brazing, adhesive, and/or crimping. In this manner, the antenna sections are “locked” together by the tensile force of inner coaxial conductor 150 and/or dielectric 132, which may result in improved strength and stiffness of the antenna assembly.

Outer jacket 124 may be applied to the outer surface of radiating portion 12 by any suitable method, for example without limitation, heat shrinking, overmolding, coating, spraying, dipping, powder coating, baking and/or film deposition.

It is contemplated that the steps of a method in accordance with the present disclosure can be performed in a different ordering than the ordering provided herein.

FIG. 5 is a schematic illustration of a microwave system, generally shown as system 500, according to an embodiment of the present disclosure. Antenna 510 is used to ablate tissue on patient “P”. Antenna 510 is coupled to coax cable 520 which has a center conductor 522 and an outer sheath 524. Outer sheath 524 is coupled to the ground terminal of microwave generator 530 which includes a microwave source 535. Outer sheath 524 is also coupled to RF generator 540 via a transmission line 526. RF generator 540 includes an RF source 542 and sensors 544. Diplexer 550 is coupled between center conductor 522 and RF generator 540. Microwave generator 530, RF generator 540 and diplexer 550 may be provided as separate units or provided in generator 28 as shown in FIG. 1.

RF generator 540 outputs RF energy to the diplexer 550 which combines the RF energy with the microwave energy outputted by microwave generator 530. Diplexer 550 implements frequency domain multiplexing where two ports are multiplexed onto a third port. The diplexer 550 blocks the RF energy from getting into the microwave generator 530 and blocks microwave energy from getting into the RF generator 540. Diplexer 550 allows both the RF energy and the microwave energy to flow to antenna 510 simultaneously through center conductor 522. The RF energy is outputted from the antenna to the ablation zone 610 as depicted in FIG. 6. The RF path is depicted by lines 620 in FIG. 5B. The RF energy flows from antenna 510 to the ablation zone 610 and uses the same return path as the microwave energy by utilizing the outer sheath 524.

The RF energy returning from the ablation zone is used as a feedback signal and is provided to the RF generator 540 via transmission line 526. The return RF energy is combined with the RF energy provided by the RF source 542. The combined RF energy is then provided to sensors 544 which measure the voltage, current and phase of the RF energy. A sensor may be operable to measure at least one parameter of the second energy generated by the second energy source and the second energy returned from the region of tissue. Such sensors are within the purview of those skilled in the art. The measured voltage, current, and/or the phase of the RF energy, is provided to a controller 560 which calculates the tissue impedance based on the measured voltage and current. Based on the tissue impedance, the controller 560 controls the output of the microwave generator 530. Alternatively, the controller 560 can calculate the tissue impedance and display the value on a display (not shown) so that a user may control the output of the microwave generator 530 or the controller 560 can automatically adjust the output of the microwave generator by comparing the calculated tissue impedance to a predetermined impedance stored in the controller.

The controller 560 may include a microcontroller operably connected to a memory, which may be volatile type memory (e.g., RAM) and/or non-volatile type memory (e.g., flash media, disk media, etc.). The microcontroller includes an output port that is operably connected to the microwave generator 530 allowing the microcontroller to control the output of the microwave generator 530. Those skilled in the art will appreciate that the microcontroller may be substituted by any logic controller (e.g., control circuit) adapted to perform the calculations discussed herein

In another embodiment according to the present disclosure, and as shown in FIG. 7, an electrosurgical system 700 is provided with an RF return pad 710 to receive a return RF signal from the ablation zone 810 as shown in FIG. 8. As shown in FIG. 8, the antenna 510 outputs RF energy in a path indicated by 820. The outputted RF energy is received by the RF return pad 810 and provided as a feedback to the RF generator 540 which is used to directly or indirectly control the microwave generator 530. The RF return pad 710 may have any suitable regular or irregular shape such as circular or polygonal. RF return pad 710 may be a conductive pad that may include a plurality of conductive arranged in a regular or irregular array. Each of the plurality of conductive elements may be equally-sized or differently-sized and may form a grid/array on the conductive pad. The plurality of conductive elements may also be arranged in a suitable spiral or radial orientation on the conductive pad. The use of the term “conductive pad” as described herein is not meant to be limiting and may indicate a variety of different pads including, but not limited to, conductive, inductive, or capacitive pads.

Although the above described embodiments describe a generator 28 having the microwave generator 530, RF generator 540, diverter 550 and controller 560, it is to be appreciated by one skilled in the art that some or all of these elements may be included in a single device or may function as separate components that are interconnected when used during an ablation procedure. Further, controller 560 may be included in the generator 28 or it may be a separate computer or laptop connected to the generator 28.

The described embodiments of the present disclosure are intended to be illustrative rather than restrictive, and are not intended to represent every embodiment of the present disclosure. Various modifications and variations can be made without departing from the spirit or scope of the disclosure as set forth in the following claims both literally and in equivalents recognized in law. 

1. A method of ablating tissue using microwave energy, the method comprising the steps of: generating a microwave energy signal with a first energy source; generating a radio frequency (RF) energy signal with a second energy source; generating a multiplexed energy signal by multiplexing the microwave energy signal and the RF energy signal and outputting the multiplexed energy signal to a tissue region; receiving a portion of the RF energy signal returned from the tissue region; measuring a parameter of the portion of the RF energy signal returned from the tissue region; and controlling an output level of the microwave energy signal based on the measured parameter.
 2. The microwave ablation method according to claim 1, wherein the measured parameter is voltage, current and/or phase.
 3. The microwave ablation method according to claim 1 further comprising the steps of: measuring a voltage, current and/or phase of the portion of the RF energy signal returned from the tissue region; and calculating a tissue impedance based on the measured voltage, current and/or phase.
 4. The microwave ablation method according to claim 3 further including the step of: controlling the output level of the microwave energy signal based on the calculated tissue impedance.
 5. The microwave ablation method according to claim 3, further including the steps of: displaying the calculated tissue impedance.
 6. The microwave ablation method according to claim 1, further including the step of: providing an RF return pad configured to receive the portion of the RF energy signal returned from the tissue region.
 7. A method of ablating tissue using microwave energy, the method comprising the steps of: generating a microwave energy signal with a first energy source; generating a radio frequency (RF) energy signal with a second energy source; generating a multiplexed energy signal by multiplexing the microwave energy signal and the RF energy signal and outputting the multiplexed energy signal to a tissue region; receiving a portion of the RF energy signal returned from the tissue region; measuring a parameter of the portion of the RF energy signal returned from the tissue region; measuring a voltage, current and/or phase of the portion of the RF energy signal returned from the tissue region; and calculating a tissue impedance based on the measured voltage, current and/or phase, and displaying the calculated tissue impedance.
 8. A method of controlling the output of a microwave ablation system, the method comprising the steps of: providing a microwave ablation system including: a generator with a first energy source, a second energy source and a diplexer, the diplexer operable to multiplex a first energy signal from the first energy source and a second energy signal from the second energy source, wherein said second energy source is a radio frequency (RF) generator; a cable including a center conductor and an outer sheath, the multiplexed energy being transmitted through said center conductor; an antenna operable to receive the multiplexed energy from said center conductor and to deliver the multiplexed energy to a region of tissue, wherein said outer sheath acts as a return path of said second energy to said second energy source; and a sensor operable to measure at least one parameter of said second energy generated by said second energy source and the second energy returned from said region of tissue, controlling an output level of the first energy signal based on the measured parameter.
 9. The method according to claim 8, wherein the measured parameter is voltage, current and/or phase.
 10. The method according to claim 8 further comprising the steps of: measuring a voltage, current and/or phase of the portion of the RF energy signal returned from the tissue region; and calculating a tissue impedance based on the measured voltage, current and/or phase.
 11. The method according to claim 10 further including the step of: controlling an output level of the first energy signal based on the calculated tissue impedance.
 12. The method according to claim 11, further including the steps of displaying the calculated tissue impedance.
 13. The method according to claim 8, further including the step of: providing an RF return pad configured to receive the portion of the RF energy signal returned from the tissue region.
 14. A method of controlling the output of a microwave ablation system, the method comprising the steps of: providing a microwave ablation system including: a generator including a first energy source, a second energy source and a diplexer, said diplexer operable to multiplex a first energy from said first energy source and a second energy from said second energy source, wherein said second energy source is a radio frequency (RF) generator; a cable including a center conductor and an outer sheath, the multiplexed energy being transmitted through said center conductor; an antenna operable to receive the multiplexed energy from said center conductor and to deliver the multiplexed energy to a region of tissue; a return pad operable to receive said second energy outputted from said antenna, said return pad transmits said second energy to said second energy source; and a sensor operable to measure at least one parameter of said second energy generated by said second energy source and the second energy returned from said return pad; controlling an output level of the first energy signal based on the measured parameter.
 15. The method according to claim 14, wherein the measured parameter is voltage, current and/or phase.
 16. The method according to claim 14 further comprising the steps of: measuring a voltage, current and/or phase of the portion of the RF energy signal returned from the tissue region; and calculating a tissue impedance based on the measured voltage, current and/or phase.
 17. The method according to claim 16 wherein the output level of the microwave energy signal generated by the first energy source is based on the calculated tissue impedance.
 18. The microwave ablation method according to claim 17, further including the steps of: displaying the calculated tissue impedance.
 19. The microwave ablation method according to claim 14, further including the step of: providing an RF return pad configured to receive the portion of the RF energy signal returned from the tissue region. 